(This article has a real-world perspective)
A crossover is where a story includes two or more distinct characters, settings or universes.
Swiftfire has had several crossovers with other fan fiction series. The most notable was in 2001 with the Star Trek: Unity crossover that saw several series come together to deal with a specific event, an invasion by the Vendoth and then their attack on Earth.
"There's No Angels Here" was originally a crossover into the Star Trek: Pendragon universe. In 2013 the story was moved into a new universe inspired by Pendragon due to the Pendragon's author no longer being active.
Back in 2001 there were also plans to create a crossover between Star Trek: The Legend Continues and SWF. However, the idea never got past the planning stages.